The present invention relates generally to apparatus and methods for measuring the distance between two points, and particularly, to a system and method for measuring the distance from a first point spaced away from a surface of an object to a second point on a surface of an object. More specifically, the present invention relates to a remote center range finder suitable for use in radiation oncology for measuring the distance of a radiation source to the body of a patient (i.e., source-to-skin-distance) during radiation treatment.
Radiation oncology uses radiation therapy for the treatment of cancerous tumors in a patient's body. Conventional radiation therapy employs a linear accelerator or LINAC, which directs a beam of radiation (e.g., gamma ray or X-ray radiation) toward a cancerous tumor in a patient to deliver a predetermined dose of therapeutic radiation to the tumor. Unfortunately, healthy tissue and organs are often in the path of a radiation beam during radiation treatment, and may be damaged by the radiation. Therefore, it is desirable to minimize the amount of radiation delivered to healthy tissue surrounding the cancerous tumor during the course of radiation therapy.
One method for minimizing damage to healthy tissue and organs during radiation treatment is to determine the distance between the radiation source and the patient's skin along the principle axis of the radiation treatment device (i.e., along the imaginary line connecting the radiation source to the machine isocenter). This distance is typically referred to as the source-to-skin distance (SSD). Accurately measuring SSD helps ensure the radiation beam is substantially directed at the cancerous tumor's center. Accordingly, small variations in SSD measurement may cause significant variations in the radiation treatment dose to the tumor.
Current treatment plans are designed under the assumption that SSD measurement errors may occur that result in misdelivery of radiation. Treatment plans compensate for this potential misdelivery by specifying lower doses or smaller beam shapes (e.g., beams that do not radiate the edges of a tumor) than would be specified if misdelivery was not expected. Such compensation can be decreased as margins of error in SSD measurements are decreased. Consequently, improving the accuracy of SSD measurements may allow for the use of more aggressive treatments. Specifically, if the margin of error in SSD measurement is known to be very small, treatments may be designed to safely radiate a greater portion of a tumor with higher doses than would be possible where the margin of error is larger.
Treatment apparatus used in radiation oncology are typically equipped with an optical range finder that enables the user to read SSD during patient setup. For example, it is known to project a scale onto the skin surface of the patient by means of a projector. The scale is in the form of a graduated line projected onto the scene. This line must include the axis along which the distance measurement is desired, i.e. the axis connecting the first and second points. Two additional planes of light project two light stripes onto the scene. Each plane of light contains the axis. In short, three planes (one containing the scale, and two planes of light) are projected, and these planes intersect at the principle axis of the system. Numerical values applied to the scale indicate the distance of the skin surface from the focus in the intersection reticle of the light-beam localizer projected onto the skin surface. However, these devices do not directly measure SSD and may not be accurate. As a result, it is desirable to have a system and apparatus that directly measures SSD, thereby improving accuracy and precision of SSD measurements.